This invention relates to watch cases, and particularly to a wrist watch case of the type having two or more parts including a glass. It is the result of research intended to improve watch case design from various points of view: reduction of the overall thickness of the case, greater attractiveness, increased invulnerability, especially as regards shock-resistance, and simplified structure.
The tendency toward reduction of the overall thickness of watches is currently inciting designers to use flat glasses of mineral material and to make them as thin as possible. The limitations in this respect depend upon the heat- und shock-resistance of the glass.
On the other hand, research with a view to making the cases more attractive is directed to producing glasses of hard mineral material which extend all the way to the periphery of the case, so that their whole edge is visible. This poses the problem of bonding the mineral material to the annular part intended to support it, the bezel or case body. This latter component may be either of metal or of some other relatively hard material and is generally opaque.
It has already been proposed in the art of watch-making to use shock-absorbing elements in the form of vulcanizable elastomeric rings placed between two metal rings and bonded to the latter by vulcanization in situ. A design of this type is described in Swiss Pat. No. 499,148, for example. The technique of joining an elastomeric component to a rigid support, especially of metal, is well known. The surfaces of the two components to be assembled are coated with organic binders in solution, suitable for the materials of which the components are made. The latter are placed in a mold at vulcanizing temperature. The elastomer is heated to flow temperature, then forced through one or more ducts into the space between the two components to be assembled and vulcanized along with the organic binders at a temperature between 200.degree. and 300.degree. C., depending upon the elastomer used. Vulcanization gives the elastomer its final structure, particularly its elasticity, while bringing about a very effective interfacial adherence between the elastomer and the supports.
The tensile strength obtained is on the order of 50-100 kg/sq.cm. Suitable elastomers are, for example, nitrile, butyl, neoprene, polyurethane, etc.
The same application can be envisaged with a plastomer as well. The thickness of the joint thus made adherent must be on the order of 0.3 to 0.5 mm; with polyurethane, the thickness may even be about 0.1 mm.
The vulcanization of elastomeric elements on metal supports has also been disclosed in connection with watch cases. According to U.K. Published Application No. 2,039,099, gaskets are bonded by this technique on shoulders of the parts supporting them; and French Published Application No. 2,463,437 describes a method consisting in lining the inside of the caseband of a watch case with a joint adhering to the caseband by vulcanization.
Yet these prior art applications did not represent an improvement from the point of view of either reduction of the thickness of the case or attractiveness or simplification. It has now been realized, however, that vulcanization yields reliable bonds on metallized glasses and that the strength obtained is compatible with the requirements to be met by watch cases.